The present invention relates to a dispersing stabilizer for suspension polymerization of vinyl compounds, particularly vinyl chloride.
It is essential to employ various kinds of dispersing stabilizers in suspension polymerization of vinyl chloride or mixtures of vinyl chloride and other monomers copolymerizable therewith, and polyvinyl alcohols and cellulose derivatives have been employed as a dispersing stabilizer. Many kinds of polyvinyl alcohols have been proposed for the purpose, and it is known that the physical and chemical properties of polyvinyl alcohols such as an average degree of polymerization and average degree of hydrolysis exert a delicate influence upon the properties of vinyl chloride resins. However, in case of employing conventional polyvinyl alcohols singly, it is difficult to prepare polyvinyl chloride resins, particles of which have a uniform and high porosity. On the other hand, polyvinyl alcohols containing carbonyl group are known, and are commercially available under the registered trademark "Alcotex" made by Revertex Limited as a dispersing stabilizer for suspension polymerization of vinyl chloride. Japanese Patent Unexamined Publication Nos. 9488/1974 and 53270/1974 also disclose polyvinyl alcohols containing carbonyl group suited for use in the suspension polymerization. It is known that the production of coarse particles of vinyl chloride resins is decreased and also the particles of vinyl chloride resins have a proper porosity by the introduction of carbonyl group into polyvinyl alcohol. Also, according to the above Publication No. 9488/1974, the polyvinyl alcohols containing about 0.03 to about 0.15 % by mole of carbonyl group (namely an absorbance of a 0.1 % by weight aqueous solution thereof being 0.05 to 0.3 at a wavelength of 280 m.mu.) are desirable as a dispersing stabilizer, and both of those having a carbonyl group content of less than 0.03 % by mole and of more than 0.15 % by mole are undesirable by the reasons that in case of the former, they have no effect, and in case of the latter, polyvinyl alcohol in itself is remarkably colored by the introduction of a large amount of carbonyl group.
Further, while these carbonyl group-containing polyvinyl alcohols have the advantage as mentioned above, it has become apparent, according to the investigation of the present inventors, that there are some problems as stated below, when they are employed singly as a dispersing stabilizer. (1) Vinyl chloride resins are lacking in sharpness of particle size distribution, (2) scale of vinyl chloride resins is easy to adhere to a polymerization vessel, and (3) large quantities of the dispersing stabilizer are required to keep the polymerization stability.